Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
A lithographic process, as described above, is performed to selectively remove portions of a resist material overlaying the surface of a wafer, thereby exposing underlying areas of the specimen on which the resist is formed for selective processing such as etching, material deposition, implantation, and the like. Therefore, in many instances, the performance of the lithography process largely determines the characteristics (e.g., dimensions) of the structures formed on the specimen. Consequently, the trend in lithography is to design systems and components (e.g., resist materials) that are capable of forming patterns having ever smaller dimensions.
Inspection processes based on optical metrology are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including reflectometry, ellipsometry, and scatterometry implementations and associated analysis algorithms to characterize device geometry have been described. However, it remains a challenge to preserve a small measurement box size. A small measurement box size is especially important in semiconductor inline product metrology where the area available for metrology targets is minimal. The measurement box size refers to the minimum area on the specimen where measurement results are stable and not affected by edge effects (e.g., due to optical diffraction wings) in optical metrology. Hence, the smaller the measurement box size, the smaller the area required for metrology targets. In the semiconductor industry, where wafer space allocated to metrology targets is limited (often, within the scribe line or even within die), the desired box size specification can be often very challenging, such as 30 μm×30 μm, 10 μm×10 μm, or even smaller.
To shrink the size of the measurement box size the amount of signal information that arises from the area surrounding the measurement target and reaches the detector must be minimized. To minimize undesirable signal contamination, the illumination light must be projected onto the measurement target with a minimum of spillover outside of the measurement target area.
Diffraction, aberration, image quality, and other limiting effects must be controlled to achieve a smaller illumination spot size. Despite existing approaches designed to control measurement box size, achieving a small measurement box size specification over the full measurement range is very challenging.
Many optical based measurement systems employ rotating polarizer elements to manipulate the polarization of illumination light provided to a specimen, light collected from the specimen, or both. In practical systems, the input and output faces of polarization optics are not perfectly parallel. This misalignment is commonly referred to as a wedge. In addition, the mechanical bearings employed to constrain the rotational motion of the polarization optics have finite concentricity and runout errors. This causes the polarization optics to wobble about the optical axis of a beam passing through the polarization optics. Wedge errors and rotary bearing errors change the optical path of the beam relative to other optical elements in the system. This manifests itself as beam positioning errors at various critical locations in the optical path. For example, for a spectroscopic ellipsometer system, wedge errors and rotary bearing errors cause misalignment of the optical beam with a polarizer slit, the specimen under measurement, and a spectrometer slit as polarizing elements are rotated. As the measurement spot moves during rotation of polarizing elements, measurement precision, accuracy, and matching among multiple tools suffers.
In an attempt to mitigate these problems, various solutions have been contemplated. In some examples, polarizing optics are manufactured with very small wedge tolerances. However, there are practical, manufacturing limits on achievable wedge error tolerance, particularly within reasonable cost. In addition, even if it were possible to manufacture a polarizer element with zero wedge error, changes in environmental conditions (e.g., temperature) cause the wedge angle to change, resulting in movement of the measurement spot during rotation of the polarizer element.
In some examples, improved rotary bearings are employed to reduce measurement beam movement induced by bearing wobble. Again, there are practical, manufacturing limits on achievable runout error tolerance, particularly within reasonable cost. In addition, even if it were possible to manufacture bearings with perfect concentricity and zero runout, bearing wear causes increasing error over time, particularly over the lifetime of semiconductor metrology tool subject to near constant use.
In some examples, a polarizer optic located in the beam path of a converging beam is tilted to compensate for the wedge error. The tilt of the polarizer optic causes a linear offset of the output beam. At the focal point of the converging beam, the linear offset cancels the angular offset caused by the wedge. This approach is not effective if the beam passing through the polarizer optic is collimated. Also, this approach does not work as well if there are two rotating polarizer optics in the beam path. In addition, this approach is not effective when bearings wear over time or wedge errors change with temperature.
As lithographic and metrology systems are pressed to higher resolutions, measurement box size becomes a limiting factor in maintaining device yield. Thus, improved methods and systems for achieving a small measurement box size associated with a variety of metrology technologies are desired.